THE WORK DEALS WITH THE PROBLEM OF THERMAL FATIGUE IN RELATION TO THE LIFETIME PREDICTION OF THE FUSION REACTOR FIRST WALL. THE AIM IS TO COMPARE DIFFERENT LIFETIME METHODOLOGIES AMONG THEM AND WITH EXPERIMENTAL RESULTS. TO FULLFIL THIS PURPOSE IT WAS NCESAARY TO DEVELOP A NEW NUMERICAL METHODOLOGY CALLED REDUCED-3D, ESPECIALLY SUITABLE FOR THERMAL FATIGUE PROBLEMS.